STEPPING out onto the glass platform of the Willis Tower, 412 metres above the streets of Chicago is enough to make most people dizzy. Not so babies, who are born with no fear of heights. Now it seems that this wariness develops as a result of crawling.

You might think fear of heights would be innate, since falling from high up can result in injury or death. But babies with little experience of crawling are not afraid of heights. “Mothers almost universally report that their babies go through a phase wherein they will go over the edge of a bed or a changing table if a caregiver doesn’t intervene,” says Joseph Campos at the University of California in Berkeley, who supervised the research. Then suddenly, six weeks or so after they learn to crawl, they seemingly become scared.